Image copyrightPress trust of IndiaImage caption
The boat sank off Port Blair, the capital of the Andaman Islands, reports said

A tourist boat has sunk off India's Andaman and Nicobar Islands in the Bay of Bengal, with the loss of 21 lives, officials say.

The boat, Aqua Marine, capsized on Sunday afternoon off Port Blair, the capital of the islands.

The Indian air force and the coast guard have rescued 29 people, authorities say. One person is still missing.

Most of those on board are believed to be Indians from Tamil Nadu and Mumbai.

An investigation is looking at the cause of the sinking amid reports the Aqua Marine was carrying too many people when it capsized.

Prime Minister Manmohan Singh "expressed shock over the boat tragedy", said his office in a statement.

Mr Singh said he had asked India's national agencies to help in the rescue and relief operation.

Those rescued have been taken to hospital in Port Blair, with "several of them seriously injured", an official told AFP news agency.

"There were three children on board. One child is dead,'' local deputy commissioner P Jawahar told BBC Hindi.

'People all around'

Some of the survivors have been recounting their ordeal.

"Twenty minutes after the journey had started, the boat, which was overcrowded, started to tilt," said Thiagarajan, 63, who lost his wife, brother, uncle and aunt in the accident.

He says he saw water entering the boat from the rear and within 15 minutes, the boat started to capsize.

"I don't know swimming, and I went down with the boat when it capsized, but then I pushed and managed to reach the surface and saw something floating and held it. I could see people all around. I can only remember me holding my wife's sari.

"And then I saw Prabhu, the boat's master, swimming towards me and he caught me and took me to a nearby boat. I am so grateful to all who saved me or I would have also been dead like my kin," he said.

With tears welling up in his eyes, he adds: "I cannot think about all that I have lost."

Every year thousands of tourists visit the coral-reef fringed Andaman islands, which form the most easterly part of Indian territory.

The archipelago of hundreds of islands sits at least 1,000 km (600 miles) from the mainland, and is closer to the coast of Burma (Myanmar).